1. Field of the Invention
The present invention relates to an optical encoder and an optical lens module.
2. Description of the Related Art
At present, a so-called encoder of an optical type, a magnetic type or the like for detecting a rectilinear displacement amount in a machine tool stage, a three-dimensional measuring instrument and the like, or for detecting a rotational angle in a servomotor and the like, is used.
The optical encoder is generally composed of a scale fixed to a member for detecting the displacement of the stage or the like, and a sensor head for detecting the displacement of the scale. The sensor head has a light emitting portion for irradiating light on the scale, and a light receiving portion for detecting a light beam modulated by the scale, and the movement of the scale is detected in accordance with an intensity change of a received light beam.
As a first prior art, a typical optical encoder will be described with reference to FIG. 33. FIG. 33 is a constitution diagram showing an optical encoder according to a prior art, using a surface emitting laser and a reflective type scale.
This optical encoder using a surface emitting laser and a reflective type scale is disclosed, for example, in U.S. Pat. No. 6,713,756.
As shown in FIG. 33, this encoder is configured by a reflective type scale 20 and a sensor head 30. A displacement amount detecting optical pattern 23 and a reference position detecting optical pattern 24 are formed on the surface of the scale 20, and these patterns are formed by patterning a metallic thin film such as chromium or the like on the surface of a transparent member such as a glass. In the sensor head 30, a displacement amount detecting photodetector 37 and a reference position detecting photodetector 39 are formed on a semiconductor substrate 34, a surface emitting laser 32 is disposed on the semiconductor substrate 34, and the positional relationship of a light source 32, and the photodetectors 37 and 39 is kept constant.
The scale 20 is interlocked with a stage (not shown) or the like, and moves relatively in the direction of the arrow of FIG. 33 with respect to the sensor head 30, and the sensor head 30 detects a movement amount, a moving direction, and a reference position thereof on the basis of an intensity change of the light beam modulated by the scale 20. The output signals from the sensor head are output as waveforms, for example, as those of FIG. 35. Here, A-phase and B-phase are waveforms which are output along with the movement of the scale 20, and are generally quasi sinusoidal waves. Further, Z-phase is a signal to be output when a reference position is detected. The A-phase and the B-phase are signals deviating from each other by 90 degrees in phase, and it is possible to detect the moving direction of the scale 20 on the basis of the phase relationship between the signals of the A-phase and the B-phase.
In this prior art, because the scale 20 displaces with respect to the sensor head 30 while maintaining a positional relationship by which a so-called Talbot image can be formed, a bright/dark pattern similar to a periodic pattern of the scale 20 is projected on the movement amount detecting photodetector 37, and the bright/dark pattern moves on the photodetector 37 along with the movement of the scale 20.
The Talbot image will be described by using FIG. 34. Here, the description will be carried out by supposing a transmission type encoder in order to simplify the description. However, the completely same argument is achieved with respect to the reflective type encoder as well.
As shown in FIG. 34, respective constituting parameters will be defined as follows.    z0: distance between the light source 1 and the surface of the scale 2 on which diffraction grating is formed    z2: distance between the surface of the scale 2 on which diffraction grating is formed and the photodetector 3    p1: pitch of the diffraction grating on the scale 2    p2: pitch of the bright/dark pattern to be projected on the light receiving surface of the photodetector 3
In accordance with optical diffraction theory, when z0 and z2 defined as described above are in or close to a specific relationship satisfying the relationship shown by the following equation (1), a bright/dark pattern similar to the diffraction grating pattern of the scale 2, i.e., a Talbot image is formed on the light receiving surface of the photodetector 3:(1/z0)+(1/z2)=λ/k(p1)2  (1)where, λ denotes a wavelength of a light beam emitted from a light source, and k denotes an integer.
In this case, the pitch p2 of the diffraction interference pattern on the light receiving surface can be determined by the following equation (2).P2=p1×(z0+z2)/z0  (2)
When the scale 2 is displaced in the pitch direction of the diffraction grating with respect to the light source 1, the bright/dark pattern projected on the photodetector 3 moves in a displacement direction of the scale 2 with the same space period being kept.
Therefore, provided that a period p20 of a light receiving portion 4 of the photodetector 3 is set to the same value as that of the pitch p2 of the bright/dark pattern determined by the equation (2), because a periodic intensity signal is obtained from the photodetector 3 every time the scale 2 moves by p1 in the pitch direction, the displacement amount of the scale 2 in the pitch direction can be detected.
Returning to FIG. 33 to continue the description, because the surface emitting laser light source 32, and the periodic optical pattern 23 and the photodetector 37 are disposed so as to have a positional relationship in which the above-described Talbot image can be formed and detected, the bright/dark pattern similar to the periodic optical pattern 23 formed on the scale 20 is projected on the photodetector 37. The period of the bright/dark pattern is the period p2 calculated by equation (2), and the photodetector 37 is formed so as to have the period of this p2. Accordingly, a movement of the bright/dark pattern can be detected by the photodetector 37.
Next, a second prior art will be described with reference to FIG. 36. FIG. 36 shows an encoder according to the prior art, in which the light source 1 is disposed at the side opposite to the scale with respect to the semiconductor substrate 5 on which the photodetector 3 is formed.
This encoder in accordance with the second prior art is disclosed, for example, in U.S. Pat. No. 6,603,114.
As shown in FIG. 36, in this encoder, the photodetector 3, a slit 100, and a blind hole 1000 are formed at a semiconductor substrate 5, and the light source 1 is disposed at the side opposite to the scale 2 so as to provide the slit 100 therebetween. The slit 100 is disposed at the depth of the blind hole 1000. This slit 100 is formed from a metallic film 74, and the metallic film 74 is sandwiched from the upper and lower portions by translucent films 76 such as a silicon oxide film.
It is configured such that the light beam emitted from the light source 1 passes through the translucent portion of the slit 100 formed in the depth of the blind hole 100, and irradiates the scale 2, and a signal light modulated by the scale 2 is detected by the photodetector 3 formed on the semiconductor substrate 5. Accordingly, it is configured such that a movement of the scale 2 can be detected on the basis of an intensity change of the signal light.
Because the optical encoder has the features of being highly precise, having a high resolution, being a non-contact type, and being superior in resistance to electromagnetic radiation problems, the optical encoder is utilized in various fields, and especially an encoder requiring high precision and high resolution is mainly of an optical type.